The present invention relates to thermography or raised printing, and in particular to methods and apparatus for applying heat to printed sheets.
Thermography, or raised printing, is carried out by applying a thermography powder, such as a resin based powder, to a printed sheet while the ink is still tacky. Sufficient heat is applied to melt the powder, whereupon the powder expands and thus imparts a "raised" effect to the printed areas.
Thermography apparatus which has heretofore been employed for carrying-out raised printing operations has usually included a feeder section into which the printed paper sheets are fed. Resinous powder, e.g., a nylon resin, is applied to the printed surface and adheres to the stilltacky ink. Thereafter, excess powder is shaken or sucked from the sheet, leaving powder on the ink only. The sheets are then placed onto a conveyor belt which conveys the sheets through a heat tunnel, with the printed side facing upwardly. An overhead heater, such as an electric resistance heater, heats the sheets and powder to the melting temperature of the powder as the sheets travel through the heating tunnel. As the powder melts, it expands and thus "raises" the printing. Thereafter, the conveyor may conduct the sheets through a cooling tunnel where cooling air is blown against the sheets to solidify the raised print.
Although equipment of this type has been commercially successful, certain problems have occassionally arisen in connection with the heating step in that the sheets sometimes tend to curl when heated. This tendency exists especially in cases where the sheets are damp and the top surface of the sheet dries at a faster rate than the bottom surface. Accordingly, the top surface contracts at a faster rate than the bottom surface, causing the sheet to curl. Curling can also occur in connection with laminated sheets as top and bottom surfaces deform at different rates.
Efforts have been made to mitigate against the curling problem by employing sheets that have been cut such that the grain runs lengthwise. While this practice may reduce the degree of curling to some extent, it is not always practicable or possible to carry out in all cases.
Reliance has been placed upon a vacuum hold-down action in the cooling tunnel in order to straighten-out curled sheets, but this is not always successful and dictates the need for a cooling tunnel even in cases where otherwise not required. Also, the height of the inlet opening of the cooling tunnel must be raised in order to receive curled sheets; this raising of the inlet height reduces the cooling efficiency.
It is, therefore, an object of the present invention to minimize or obviate problems of the above type.
Another object is to provide novel methods and apparatus for applying heat to the sheet in a manner which greatly reduces the sheet-curling problem.
An additional object is to enable the length of a raised printing machine to be reduced, while increasing the rate of production that can be achieved thereby.